Getting specific about what we "call" it is an exercise in marketing psychology. "3.5," after all, was coined in an era where tech language was starting to be embraced by the culture at large. "Essentials" was 4e's appeal to folks who felt its design wasn't true to D&D's roots. "One D&D" seems like it's meant to sooth concerns that this is a "different game," to reduce the friction felt by the backwards compatibility crowd.
You could call this the "anniversary edition," since it's coming on a big milestone for the brand, and it's including a lot of references to older versions of D&D.
Whatever you decide, the root doesn't change. Almost every edition has gone through a midlife crisis facelift. Because WotC makes most of the money on core rulebooks, a core rulebook refresh is a way to prime the coffers a bit whenever sales start to lag. That usually reignites some interest in the next few books they put out (gotta update the artificer and the transmuter and the grave domain and monsters X, Y, and Z, too, etc.).
It'll be real nice if this helps to delay "6e" by another 10 years. Being able to play a 2014 adventure in 2034 and to still have it basically work is a pretty good accomplishment. Maybe we'll be ready for a more substantial overhaul of the system then. Even if it's more like 5-6 years, still a pretty good run.
It'd be kind of awesome if, in those 10 years, they find out a way to get off of the edition treadmill, but because the edition treadmill makes money, and might even be good for the game from a design perspective, maybe it's OK if they stay on it, too.